Carbon nanotubes are seamless tubes of graphite sheets with full fullerene caps. Carbon nanotubes are made in the presence of transition metal catalysts, and were first discovered as multilayer concentric tubes, usually referred to as carbon multi-walled nanotubes, and subsequently as carbon single-walled nanotubes (SWNTs). Carbon nanotubes are useful for a wide range of applications including nanoscale electronic devices, high strength materials, electron field emission, tips for scanning probe microscopy, and gas storage. Generally, SWNTs are preferred over multi-walled carbon nanotubes for use in these applications. However, the availability of SWNTs in large enough quantities necessary is still problematic, and methods for their large scale production are still needed.
A typical method for the synthesis of SWNTs is described in U.S. Pat. No. 6,955,800 to The Board of Regents of University of Oklahoma where supported catalytic particles are reduced in a fluidized bed and carbon nanotubes are catalytically formed by exposing the reduced catalytic particles to a carbon-containing gas in the fluidized bed. The reacted catalytic particles are cooled and the catalytic material is separated from the carbon nanotubes. The synthesis of SWNTs is inefficient since the reported yield is about 1 g/h.
U.S. Pat. No. 6,905,544 describes a method for continuously manufacturing carbon nanotubes. A catalyst having a specific gravity and particle diameter is placed on a fluidized bed and maintained within a predetermined temperature and pressure range to form the carbon nanotubes. Thus, the fluidized bed containing the catalytic particles acts as the reactor for the synthesis of carbon nanotubes.
Another method, described in U.S. Publication No. 2005/0074392 grows single-wall carbon nanotubes using iron and molybdenum catalysts supported on magnesium oxide support material. The supported catalyst is contacted with a carbon-containing gas to make single-wall carbon nanotubes. The process can be conducted in batch, continuous or semi-continuous modes, in reactors, such as a transport reactor, fluidized bed reactor, moving bed reactors and combinations thereof.
U.S. Publication No. 2004/0151654 to Tsinghua University, China describes a method for the continuous manufacture of carbon nanotubes using a nano-agglomerate fluidized-bed and reactor. The publication describes a method where a transition metal oxide on a support is activated by flowing a mixture of nitrogen and hydrogen or carbon monoxide into the reactor to reduce the nanosized transition metal oxide particles to nanosized metal particles and the catalyst is in the form of nano-agglomerates. The nano-agglomerate catalyst is transported into a fluidized-bed reactor and reacted with a gas of lower hydrocarbons having less than 7 carbon atoms to form carbon nanotubes.
The currently available methods for the continuous synthesis of SWNTs have production rates of less than 1 g/h. A major shortcoming of the current continuous large scale production is the injection of catalyst particles into the reactor. Thus, there is a need for methods for the continuous production of high quality SWNTs on an industrial scale.